Abstract: A means for detecting helium which has flowed into a system by substitution for air at a leaking location and mixed with the remaining gases, and after the helium gas permeates a helium permeable membrane as a sample gas so that the air component unnecessary for detection is suppressed, the helium gas enters the space of a mass spectrometer tube. That is, the membrane suppresses the permeation of the air component and selectively allows the helium component to pass therethrough, so that the helium component ratio in the space of the spectrometer tube is made larger than that in the sample gas. The helium partial pressure in the gas with thus increased helium component is determined by the mass spectrometer.The change in the helium partial pressure in the spectrometer tube is produced depending on the substitution between the air component gases and the helium gas at the leak location.